The present invention relates to a process for the cross-linking of high density polyethylene in solid state.
Is is known that by the cross-linking process some mechanical properties of the polyethylene, such as the stretching resistance under constant load and the tensile strength in the presence of surface active agents are improved. Moreover, the cross-linked polyethylene shows a better resistance to the chemical agents and, when subjected to high temperatures (greater than 150.degree. C.), maintains the initial shape, even if all the main properties are lost, since the cross-linking prevents creep from occurring.
All these advantageous properties of the cross-linked polyethylene were not fully exploited up to date since the cross-linking process, which usually employs high temperatures (&gt;145.degree. C.) and higher than the melting point of the polyethylene, is inconsistent with the most common processes for the polyethylene processing.
In fact, the cross-linking must take place when the melted material is confined in a mold, as taught by the technology of the thermosetting materials. The mold must be heated to the cross-linking temperature and then cooled to promote the removal of the molding.
Thus, amongst the several technologies for the processing of the polyethylene, only the molding, either in a press or by injection, is consistant with the cross-linking, but much more lasting cycles than the normal ones are involved.
All these difficulties are eliminated by the process described in the present invention, by which it is possible to carry out the cross-linking of polyethylene at temperatures lower than the melting point. A polyethylene product can be thus produced by the most suitable technology (for instance blowing of hollow bodies, extrusion, injection, vacuum forming, rotary molding, etc.), and then carry out the cross-linking at a temperature lower than the melting temperature, at which the product maintains its shape without distortion. Such a cross-linking process, the nature of which will be specified hereinafter, is defined in the present disclosure as solid state cross-linking, whereas according to the conventional processes the polymer must be in the melted condition.
Apart from the afore stated advantages with respect to the processing possibility, the solid state cross-linking imparts to the artifacts outstanding physical and mechanical properties with respect to the conventional melt mass vulcanization, as it was also found by H. BRODY (J. Appl. Poly. Sci. 15, 987-1005, 1971).
In the solid state cross-linking, the cross-linking is limited to the amorphous part between the crystallites, without disturbing the crystalline phase, whereas in the melt mass cross-linking, the cross-linking causes a diminution of the crystalline percentage in the final product to take place.
The maintenance of the crystallinity in the solid state cross-linking gives products showing greater rigidity (about two-fold), higher melting point, higher HDT and Vicat and lower bulking in comparison with melt mass cross-linked artifacts. Both types of cross-linking lead to products resistant to impact and to environmental stress cracking.
The two systems most commonly used for the cross-linking of polyethylene comprise the use of organic peroxides and radiations.
The cross-linking of high density polyethylene in the solid state by means of peroxides is practically impossible: if peroxides are used having high decomposition temperature (&lt;160.degree. C.), both the preparation of the polyethylene-peroxide compound, and the processing of the compound into the desired article without premature cross-linking reactions taking place can be possible, but it will be then impossible to carry out the cross-linking of the resulting article in the solid state, since at the related temperature (&lt;130.degree. C.) the peroxide is not capable of effecting the cross-linking within reasonable times:
if peroxides are used having low decomposition temperatures (&lt;130.degree. C.), the processing of the polyethylene is not possible (melting point: 135.degree. C.) without initiating the decomposition reaction of the peroxide. PA1 sulphur based systems; PA1 systems containing sulphur donors; PA1 alkylphenol-formaldehyde resins; PA1 dinitrosocompounds.
An attempt for the solid state cross-linking of high density polyethylene by means of peroxides was successfully effected by H. BRODY (J. Appl. Poly. Sci. 15, 987-1005, 1971) through the hot swelling of the molded article in a mixture of allylmethacrylate-dicumylperoxide.
Such a technology, even if entirely valid for the study of the properties of the solid state cross-linked polyethylene, has of course no possibility of use in the industrial processing.
The cross-linking by means of radiations can be instead carried out on the solid state polyethylene, but the system is effected by serious limitations, due to the costs and to the difficulties of irradiating, with a homogeneous stream of radiations, articles of complicate shape.